1. Field of the Invention
The present invention relates to an apparatus for producing a Langmuir-Blodgett film.
2. Description of the Related Art
A Langmuir-Blodgett film (LB film) is an organic film comprising a single molecular layer of the same or different organic materials or accumulated layers thereof, the thickness of the single molecular layer being determined by the length of each of molecules which constitute the single molecular layer. The term "LB film" is derived from Irving Langmuir and Katharine Blodgett who are famous researchers in this field. As employed herein the phrase "single molecular film" means the same as the phrase "monomolecular film".
The LB film is generally formed in accordance with the following procedure. First an amphipathic compound for forming the film is dissolved in an appropriate solvent. Next, a small amount of this solution is spread on the surface of a clean liquid, preferably pure water, and the solvent is evaporated or diffused into the adjacent water phase to disperse film-forming molecules on the water surface. At this time, the film-forming molecules are arranged so that the hydrophobic groups thereof face the vapor phase side, and the hydrophilic groups thereof face the water phase side. Then, a partition plate is mechanically swept on the water surface to decrease the surface area of the water surface on which the film-forming molecules are spread. As a result, the film formed by dispersion of the film forming molecules is compressed in the planar direction to increase the density, to obtain a dense single molecular film on the water surface. Next, a solid substrate is dipped and pulled up in the direction crossing the single molecular film with the density of the single molecular film on the water surface kept constant under appropriate conditions, to deposit a single molecular layer on the solid substrate. Such a dipping and pulling-up operation is repeated to deposit, on the solid substrate, a film comprising cumulative single molecular layers and having a thickness of an integral multiple of the thickness of the single molecular layer.
The density of the single molecular film on the water surface is monitored by measuring a difference between the surface tension of the clean water surface and the surface tension of a region on the water surface which is coated with the single molecular film, i.e., surface pressure. In the operation for forming the LB film as described above, the position of the partition plate must be controlled to keep the set surface pressure constant. Since the single molecular film on the water surface is transferred onto the substrate as deposition of the LB film proceeds, the partition plate must be moved in the direction to compress the single molecular film on the water surface in order to keep the surface pressure constant. As a result, the spread area of the single molecular film on the water surface decreases with time. At this time, although the single molecular film on the water surface flows to the substrate, the flow of the single molecular film on the water surface is nonuniform because the width (the length of the end surface of the single molecular film on the water surface, which contacts the partition plate) of a water bath is smaller than the width of the substrate, and the molecular film thus converges to the substrate. This state is shown in FIGS. 1A to 1C.
All FIGS. 1A to 1C are plan views of a LB film-forming apparatus comprising a water bath 11, a partition plate 12 and a surface pressure gauge (not shown in the drawing). In FIG. 1A, it is assumed that a single molecular film 14 on the water surface has already reached to desired surface pressure. In order to keep the surface pressure constant while transferring the single molecular film 14 on the water surface to a substrate 13, the partition plate 12 is moved in the direction of compression of the single molecular film 14 on the water surface. Each of Figs. 1B and 1C shows a state in the course of deposition of the LB film on the substrate. In the drawings, arrows 15 schematically show the flow (the flow direction) of the single molecular film on the water surface. As seen from the drawings, the flow of the single molecular film on the water surface strongly converges to the substrate, particularly, near the substrate, as deposition of the LB film proceeds. Consequently, for example, in a case in which the single molecular film on the water surface has a crystalline domain structure, as a single molecular film comprising a fatty acid on the water surface, the initial domain shape, which is relatively amorphous, is changed to a shape having a long axis in the direction of compression of the film near the substrate, and thus the resultant LB film also has a domain long axis in the pulling-up direction of the substrate (for example, refer to M. F. Daniel and J. T. T. Hart, Journal of Molecular Electronics, Vol. 1, pp. 97-104 (1985), M. Sugi, N. Minari, K. Ikegami, S. Kuroda, K. Saito, and M. Saito, Thin Solid Films, Vol. 178, pp. 157-164 (1989), and O. Albrecht, H. Matsuda, K. Eguchi, and T. Nakagiri, Thin Solid Films, Vol. 221, pp. 76-280 (1992)).
A single molecular film comprising a polymer on the water surface has the tendency that the polymer main chains are made parallel to the compression direction of the film by deposition on the substrate, and the resultant LB film tends to be oriented so that the polymer main chains are parallel to the direction of pulling-up of the substrate.
As described above, the LB film has unexpected anisotropy, but manifestation of such anisotropy is very important and necessary for some applications. For example, in a case in which a polymer LB film of polyimide or the like is used as a liquid crystal alignment film, the polymer main chains have previously been arranged in a direction, and thus uniform liquid crystal alignment can be achieved without rubbing (the process of rubbing a liquid crystal alignment layer in an amorphous state with rotating cloth in a direction). Also the use of a LB film having anisotropy permits formation of an optical device utilizing optical anisotropy, such as a polarizer or the like.
The LB film is conventionally produced by batch processing, as described above, and the amount of the film which can be formed by one operation is limited by the area of the single molecular film first formed on the water surface. Namely, the amount cannot be larger than the surface area of the water bath, and is actually smaller than the surface area of the water bath because the surface of the water bath is partitioned by using the partition plate. Therefore, where a film having an area larger than the single molecular film first formed on the water surface must be deposited on the substrate, excess single molecular films on the water surface (the single molecular film on the water surface which is smaller than the area of deposition on the substrate in one operation of moving the substrate upward and downward, and the single molecular film on the water surface which is present in a region where the single molecular film on the water surface cannot be compressed by moving the partition plate due to mechanical limitation of the apparatus) is removed, and then a single molecular film must be formed again on the water surface through the process comprising spreading film molecules and compressing again. This causes a problem of productivity.
Where anisotropy is introduced into the LB film, the productivity further deteriorates for the following reasons. Immediately after the single molecular film is formed on the water surface by using the apparatus shown in FIGS. 1A to 1C, substantially no anisotropy is manifested in the single molecular film on the water surface. If anisotropy is manifested, anisotropy is manifested by the operation of compressing the single molecular film on the water surface by moving the partition plate, and thus the long axis direction of molecules is parallel to the partition plate (the direction perpendicular to the flow direction 15 of the single molecular film on the water surface shown in FIGS. 1A to 1C). As described above, the mechanism for manifesting anisotropy in the single molecular film on the water surface comprises two-dimensional stress deformation of the film caused by the flow of the single molecular film on the water surface toward a specified point (in this case, the point of the depositing operation) due to deposition on the substrate. Therefore, in order to provide the single molecular film having uniform anisotropy on the water surface, a second substrate (a substrate different from the substrate used for a target sample) must be used in deposition operations to remove an appropriate amount of single molecular film on the water surface from the specified point and form a (pseudo) stationary flow of the single molecular film on the water surface. As a result, the amount of the LB film having anisotropy which can be formed by one deposition operation is reduced by an amount corresponding to the amount of the single molecular film on the water surface removed for forming the stationary flow, thereby further deteriorating productivity. Conversely, if an operation of depositing on a desired substrate is performed before the (pseudo) stationary flow of the single molecular film is formed on the water surface, the magnitude of anisotropy gradually changes as the deposition operation proceeds, and thus it is difficult to obtain uniform anisotropy.
In order to overcome the deterioration in productivity of LB film production, some methods of continuously producing a LB film are disclosed. Typical examples of such methods include the methods disclosed in O. Albrecht et al., U.S. Pat. No. 4,783,348 and Japanese Patent Laid-Open No. 8-001058, and W. Nitsch et al., Thin Solid Films, Vol. 178, pp. 140-150 (1985).
The apparatus of continuously producing a LB film disclosed by O. Albrecht et al. and the outline of the film deposition process thereof will be described below with reference to FIG. 2.
The apparatus shown in FIG. 2 comprises a water bath 20 roughly composed of three regions, i.e., a spread region S, a compression region C, and a deposition region D, a liquid (typically, pure water) serving as sub-phase water 23 continuously flowing in the direction from the region S to the region D through the region C.
In the region S, a spread solution 24 in which film forming molecules 21 are dissolved is added dropwise onto the sub-phase water 23 through a nozzle 28. Then the solvent of the spread solution 24 is removed by evaporation, and the film forming molecules 21 are continuously moved toward the region C along a water flow 29. In the region C, the film forming molecules 21 are compressed to form a single molecular film 22 comprising the film forming molecules 21 arranged in a direction over the range of from the region C to the region D. In order to uniformly continuously compress the film forming molecules 21, in the region C, a stable water flow is essential, and thus the water bath 20 is formed so that the water surface in the region C slightly slopes downward along the water flow. In the region D, a substrate 25 connected to a substrate moving mechanism 26 is repeatedly dipped and pulled up in the direction perpendicular to the water surface, as shown by an arrow 27 to continuously deposit the single molecular film 22 on the water surface on the surface of the substrate 25, to obtain a LB film.
In the above-mentioned method and apparatus, where a predetermined amount of film forming molecules has already been present in the system, the film forming molecules 21 continuously supplied are moved until they are stopped by the end of the single molecular film 22 formed on the water surface over the range from the region C to the region D. Therefore, dropwise addition of the film forming molecules 21 is accompanied with growth of the single molecular film 22 on the water surface in the upstream direction opposite to the water flow unless the single molecular film 22 on the water surface is not removed from the region C or D (including the operation of depositing on the substrate). At this time, the pressure gradient in the single molecular film 22 on the water surface is increased by frictional force generated between the single molecular film 22 formed on the water surface and the sub-phase water 23, thereby increasing the surface pressure of the single molecular film 22 on the water surface. The ultimate surface pressure in the deposition region D is determined by the level of the sub-phase water, the strength of the water flow, the water temperature, the length and the degree of the downward slope in the region C, etc. In order to make uniform the quality (surface pressure) of the single molecular film on the water surface in the region D, it is essential to uniformly compress the film-forming molecules and the single molecular film on the water surface in the region C. Therefore, the water flow in the region C is preferably a laminar flow, and the shape of the water bath 20 is designed to decrease (typically, about 2 to 8 mm) the thickness (the distance between the water surface and the bottom of the water bath) of the sub-phase water in the region C. Also, in order to obtain the stationary water flow, in the region C, the sub-phase water 23 is caused to flow into the water bath, and flow out from the region D. In order to save the sub-phase water 23 used, the sub-phase water 23 caused to flow out from the region D is generally returned to the region S by using a pump.
In order to keep the surface pressure of the single molecular film 22 on the water surface at the predetermined value, two loops are used, which include a control loop for controlling the spread rate of the film forming molecules 22, and a control loop for keeping the level of water in the region D at the predetermined value by controlling the amount of the sub-phase water 23 held in the water bath 20.
The region D is followed by a mechanism for stationarily removing the single molecular film on the water surface which is not used for deposition on the substrate. FIGS. 3A to 3C show an example of mechanisms for removing the single molecular film on the water surface. FIG. 3A is a schematic sectional view, and FIGS. 3B and 3C are top views. In the drawings, a rotatable impeller 31 has the function to move the single molecular film 22 on the water surface present in the region D toward the removal region R, and has the function as a bulkhead for preventing excess film molecules 33 sent to the removal region D from flowing backward to the region D. The excess film molecules 33 sent to the removal region R may be removed from the region R by using an aspirator 32 or the like.
The rate of removal of the single molecular film on the water surface is set so that the sum of the amount of the single molecular film on the water surface removed per unit time by such a removal mechanism and the amount of the single molecular film on the water surface deposited on the substrate is substantially equal to the amount of the single molecular film on the water surface newly produced by continuously spreading film molecules. In this case, as shown by arrows 34 in FIG. 3B, both the flow rate and direction of the flow of the single molecular film on the water surface are made uniform, and the residence time required until the single molecular film on the water surface is deposited on the substrate after the formation thereof can be kept substantially constant. The residence time required until the single molecular film on the water surface is deposited on the substrate is a factor which influences the quality of the single molecular film on the water surface (for example, in I. R. Peterson, G. J. Russell, and G. G. Roberts, Thin Solid Films, Vol. 109, pp. 371-378 (1983), it is disclosed that in a single molecular film of a fatty acid on a water surface, the viscosity of the single molecular film on the water surface increases as the residence time increases). Therefore, in order to control the quality of the LB film, it is advantageous that the residence time can be kept constant.
In the use of the above operation, where the substrate 25 is arranged so that the surface of the substrate 25 is perpendicular to the water flow 29, as shown in FIG. 3B, substantially no anisotropy is manifested in the LB film deposited on the surface of the substrate which faces the upstream side. On the surface of the substrate 25 which faces the downstream side, the flow of the single molecular film 22 on the water surface is complicated near the substrate, with some stress deformation, but high anisotropy is not manifested in the LB film deposited on this surface. Where the substrate 25 is arranged so that the surface thereof is parallel to the water flow 29, as shown in FIG. 3C, the single molecular film 22 on the water surface is just subjected to stress deformation near the substrate 25, and thus high anisotropy is not manifested in the deposited LB film.
In order to produce the LB film having anisotropy by using the continues LB film producing apparatus, a method is possibly used in which the removal of the single molecular film on the water surface is stopped so that in the region D, the flow of the single molecular film on the water surface is generated only by the operation of depositing the LB film, as in the batch process shown in FIGS. 1A to 1C. In this case, of course, the flow rate of the single molecular film on the water surface has dependency on position, and thus the residence time required until the single molecular film on the water surface is deposited on the substrate after formation thereof is strictly kept constant with difficulty. However, even in such an operation, the water flow 29 is actually present in the region D, and thus the single molecular film 22 on the water surface tends to move in the downstream direction as a whole. As a result, local stress deformation of the single molecular film on the water surface is relieved, thereby decreasing the magnitude of anisotropy manifested in the LB film.